Problem: A rectangular box has interior dimensions 6-inches by 5-inches by 10-inches. The box is filled with as many solid 3-inch cubes as possible, with all of the cubes entirely inside the rectangular box. What percent of the volume of the box is taken up by the cubes?
Three-inch cubes can fill a rectangular box only if the edge lengths of the box are all integer multiples of 3 inches.  The largest such box whose dimensions are less than or equal to those of the $6''\times5''\times10''$ box is a $6''\times3''\times9''$ box.  The ratio of the volumes of these two boxes is  \[
\frac{6\cdot3\cdot9}{6\cdot5\cdot10}=\frac{3\cdot9}{5\cdot10}=\frac{27}{50},
\] which is $\boxed{54}$ percent.